1. Field of Invention
The present invention relates to improvements relating to the industrial process employed for the direct synthesis of alkylhalosilanes.
2. Description of Related Art
The industrial process for the manufacture of alkylhalosilanes and, for example, of dimethyldichlorosilane, subsequently referred to as DMDCS, is a well known process which is described in particular in the U.S. Pat. No. 2,380,995 and in the work by Walter Noll, Chemistry and Technology of Silicones, 1968, published by Academic Press Inc., London, pages 26-41.
According to this “direct synthesis” or “Rochow synthesis” process, the alkylhalosilanes, for example DMDCS, are manufactured directly by reaction of methyl chloride with a solid contact body formed of silicon and of a catalyst comprising copper, according to the reaction:2CH3Cl+Si→(CH3)2SiCl2.
In reality, other coproducts, such as those mentioned below, are formed during the direct synthesis: other alkylhalosilanes, such as methyltrichlorosilane CH3SiCl3, subsequently referred to as MTCS, and trimethylchlorosilane (CH3)3SiCl, subsequently referred to as TMCS; halogenated alkylhydrosilanes, such as, for example, methylhydrodichlorosilane (CH3)HSiCl2, subsequently referred to as MHDCS; and heavy products which are polysilanes and in particular disilanes, such as, for example, trimethyltrichlorodisilane (CH3)3Si2Cl3 and dimethyltetrachlorodisilane (CH3)2Si2Cl4.
Among all the products obtained by direct synthesis, the dialkyldihalosilane, and for example DMDCS, is the main product, that is to say the product obtained in predominant amount. This product is highly desirable as, after hydrolysis and polymerization, it makes it possible to obtain oils and gums which are base products for the manufacture of silicones.
It is known to use copper, taken in the form of copper metal or in the form of copper-based chemical compounds, as catalyst of the direct synthesis reaction.
It is also known, for the purpose of bringing the performance of the direct synthesis to an economically viable level, to add, to the copper, a promoter combination comprising one or more promoter additive(s); these additives can be: zinc or a zinc halide (U.S. Pat. No. 2,464,033), aluminum (U.S. Pat. Nos. 2,403,370 and 2,427,605), tin, manganese, nickel and silver (British patent GB-A-1 207 466), cobalt (British patent GB-A-907 161), potassium chloride (Soviet patent SU-A-307 650) or arsenic or an arsenic compound (U.S. Pat. No. 4,762,940).
However, despite all the importance of the catalytic systems (copper catalysts as a mixture with a promoter combination) provided in the prior art, research continues in this field in order to obtain better performances than those obtained with the best catalytic systems known previously, in particular the catalytic system comprising copper, zinc and tin.
Various copper sources can be used, mainly, in addition to metallic copper (Cu°), cuprous chloride (CuCl) and oxidized copper.
CuCl is known for contributing a gain in activity or reactivity (evaluated, for example, by weight of the silanes obtained per hour and per kilogram of silicon initially involved) and in selectivity (evaluated, for example, by the percentage by weight of DMDCS formed with respect to the silanes obtained) in comparison with metallic copper. It also makes it possible to reduce the duration of the period of initiation of the reaction and also the amount of the byproducts formed during this initiation period; this is because, in order to carry out the direct synthesis reaction, there is advantageously carried out beforehand, as is well known, an initial stage of activation of the contact body (formed by the combination based on silicon+catalyst+optional promoters); one of the activation means which is highly suitable can consist in bringing said contact body to a certain temperature which can be lower or greater by a few degrees to a few tens of degrees than the temperature chosen for the direct synthesis reaction.
Numerous authors have taken an interest in this initiation period, which corresponds to the reaction between the CuCl and the silicon and which results in the formation of active sites. The mechanism of this reaction is not yet clearly defined after more than fifty years of study. Two reaction models are considered today:
Tamhankar S. S., Gokkarn A. N. and Doraiswamy L. K., 1981, Chem. Eng. Sci., 36, 1365-1372, propose a two-stage mechanism. The first stage is the reduction of the CuCl by the silicon to form metallic copper and SiCl4, followed by the formation of Cu3Si by diffusion of the copper into the silicon:nSi+4CuCl→(n−1)Si*+4Cu*+SiCl4 3Cu*+Si→Cu3Si
Weber G., Vile D., Souha M. and Guillot B., 1988, C.R. Acad. Sci. Paris, Vol. 307, Series II, pages 1155-1161, propose a reaction pathway where the metallic copper is the final product:7Si+12CuCl→Cu3Si+3SiCl4 31Cu3Si+12CuCl→7Cu15Si+3SiCl4 9Cu15Si+20CuCl→31Cu5Si+5SiCl4 Cu5Si+4CuCl→9Cu+SiCl4 
Many factors influence the reaction between Si and CuCl: the concentration of CuCl, the operations of mixing and milling the powders, the thickness of the layer of SiO2 on the silicon, the temperature and the pressure.
Recent studies targeted at determining the initial stage of the reaction between the copper and the CuCl have been published; Acker J., Köhter S., Lewis K. M. and Bohmhammel K., 2003, Silicon Chemistry, 2, 195-206. The main conclusion of these studies is that the reaction between the copper chloride and the silicon takes place in the solid state. The slightest change in the surface properties in the CuCl thus results in a modification to the reactivity.
On the basis of these facts, it may be expected that the specific surface of the CuCl used is a key parameter in its reactivity which logically directs us towards the use of fine particles which offer a high surface area for contact with the silicon.
Nevertheless, for reasons of industrial use, the Applicant Company has tested a novel shaping of this product in the form of beads. These spherical beads have a smooth surface and result from an atomization or prilling process. They exhibit the advantage of offering better flow and low dusting in comparison with powders with a low particle size. On the other hand, the Applicant Company expected to obtain mediocre results in terms in particular of reactivity and of selectivity, given their low specific surface and the absence of irregularities on the surface which are known to promote the initiation of solid/solid reactions.